1. Field of the Invention
This invention relates to optical fibres, in particular to single mode optical fibres with reduced polarization mode dispersion (PMD). This invention also relates to a method for obtaining an optical fibre with reduced PMD.
The invention may apply to all kinds of optical fibres, as for example dispersion-unshifted fibres, dispersion-shifted fibres, non-zero-dispersion fibres, dispersion-compensating fibres, fibres for optical amplifiers (such as erbium doped fibres), fibres for optical sensors, fibres for gratings.
2. Related Art
It is well known that in the so called “single mode optical fibres”, which are commonly used in optical telecommunication systems, two modes exist, with orthogonal polarizations.
If the fiber is perfectly circularly symmetric in both internal geometry and stress, the two polarization modes are degenerate and propagate with the same group velocity. That is, they have no relative time delay difference after traveling the same distance in the fibre.
In practical single mode fibres, however, various imperfections such as asymmetrical lateral stress and/or a non-circular core typically break the circular symmetry of the ideal modes. The two modes then propagate with different propagation constants (k1 and k2). The difference between the propagation constants is called “birefringence” (Δβ): the magnitude of birefringence is given by the difference in the propagation constants of the two orthogonal modes:β1=Δβ=|k1−k2|  (1)
Fibre birefringence can result from either a geometrical deformation and/or from various elasto-optic, magneto-optic and/or electro-optic refractive index changes. In so-called “polarization-preserving fibres” asymmetry is deliberately introduced into the fiber. However, in ordinary, non polarization-preserving fibres the birefringence causing mechanisms act on the fibre in a substantially unpredictable manner. Thus, the polarization state of the guided light will typically evolve through a random sequence of states along the fibre, with the polarization state at the fiber output typically being both unpredictable and unstable. On average, a given polarization state in a given fibre is reproduced after a certain length LB, which is called the “beat length” associated with the given fibre. The fibre beat length is inversely proportional to the fiber birefringence and its value is given byLB=2π/Δβ  (2)
Accordingly, the more the fiber is birefringent the shorter is the beat length and vice versa. Typical beat lengths observed in practice range from as short as 2-3 mm (high birefringence fibres) to as long as 10-100 m (low birefringence fibres). In addition to causing periodic changes in the polarization state of light traveling in a fiber, the presence of birefringence causes the two polarization modes to travel at different group velocities, the difference increasing as the birefringence increases. This effect is called “polarization mode dispersion” (PMD). The differential time delay Δτ between the two polarization modes will be referred herein and in the following as “differential group delay” (DGD). For a “short fibre section”, that is, for a section which is short enough that any perturbations acting on it can be considered as constant over its length L, a PMD coefficient may be defined as the DGD for unit lengthPMDC=Δτ/Land is usually expressed in units of picoseconds per kilometer of fibre length.
The expression “unperturbed fibre” will be used in the following interchangeably and with the same meaning of the expression “short fibre section”.
In long fibre spans, for example in a span between two optical amplifiers, in an optical telecommunication system, DGD does not accumulate in a linear fashion. Rather, because of random variations in the perturbations along a fibre span, the effects of one section of a fibre span may either add to or subtract from the effect of another section. As a result, the DGD in long fibre spans accumulates in a random-walk like process that leads to a square root of length dependence.
An important parameter for distinguishing between the short length regime, where polarization effects are deterministic, and the long length regime, where they become statistical, is the “correlation length” LC, which is often referred also as the “coupling length”. One can imagine a large population of uniformly birefringent fibres, all subjected to the same random perturbations. Into each fibre of the population a lightwave is launched such that only one of the two polarization modes is excited at the input. As the lightwave propagates down the fiber, it initially remains in the starting polarization mode. Eventually, however, the state of polarization evolves away from the initial linear state as a result of power leaking over to the other polarization mode. This leakage of power (mode coupling) occurs because of variations in the birefringence along the fiber, caused by the random perturbations. If one were to average the amount of optical power that has leaked to the orthogonal state over all the fibres of the population, one would find that this average power grows with the distance from the input, until, at some large distance, the average power in the two polarization modes is approximately the same. The correlation length LC is defined as the length at which the average power in the orthogonal polarization mode, P⊥, is within 1/e2 of the power in the starting mode, P∥, i.e.                     〈                              P                                ⁡                      (                          L              C                        )                          〉            -              〈                              P            ⊥                    ⁡                      (                          L              C                        )                          〉                    P      total        =      1          e      2      
It is remarked that the correlation length LC is extremely sensitive to the way in which the fibre is deployed, with values ranging from less than 1 m for a fiber on a spool, up to more than 1 km for cabled fibre.
PMD in conventional single mode fibres results in harmful signal distortion and is undesirable, especially for applications that involve high bit rates (e.g. equal to or greater than 10 Gbit/s) or analog transmission (e.g. for optical fibre analog CATV systems).
Various attempts to reduce the PMD coefficient in single mode optical fibres have been made. A known method of reducing the PMD coefficient involves spinning the preform during the fibre drawing process. The spinning causes the internal geometric and/or stress asymmetries of the fibre to rotate about the fibre's axis as one progresses down that axis. It is commonly believed that the reduction in PMD coefficient produced by spinning is proportional to the spin rate. Unfortunately, very high spin rates are generally required to deal with the asymmetries of typical fibres, e.g. spin rates greater than 5000 rpm. Spinning a preform at such rates is not a practical solution for commercial fibre production.
U.S. Pat. No. 5,298,047 and U.S. Pat. No. 5,418,881 to AT&T Bell Laboratories disclose that PMD can be substantially reduced if, during drawing of the fibre, a torque is applied to the fibre such that a permanent “spin” is impressed on the fibre. The torque is applied such that the spin impressed on the fibre has not constant spatial frequency, e.g., has alternately clockwise and counterclockwise helicity.
WO 97/26221 to Corning Incorporated discloses a method for reducing PMD in single mode fibres by spinning the fibre during the drawing process in accordance with a spin function having sufficient harmonic content. Examples of suitable spin functions disclosed are frequency modulated and amplitude modulated sine waves. According to the above WO patent application, the spin rate should vary both in magnitude and spatial distribution along the length of the fiber to achieve an optimum PMD reduction. When so varied, the spinning achieves transfer of energy between polarization modes (mode coupling) for a variety of beat lenghts.
The inventors have observed that by spinning the fiber using the amplitude or frequency modulated spin functions of the above WO patent application the correlation length LC of the fiber is reduced. As it is known (see, for example, F. Curti et al., “Statistical treatment of the evolution of the principal states of polarization in single-mode fibers” IEEE J. Lightwave Tech., vol. 8, pp. 1162-1166, 1990), in the long length regime (that is, when the length of the fibre L is much longer than LC) the mean value of the DGD is proportional to the square root of the correlation length: thus, a reduction of LC leads to a reduction of the DGD, but only with a square root dependence. This result is not completely satisfactory.